Controlled impedance bias circuit

ABSTRACT

A bias circuit is provided to present a controlled impedance to an input of an RF amplifier to improve linearity of the RF amplifier. The bias circuit may control the impedance such that a low impedance is presented to the input of the amplifier at low frequencies and a high impedance is presented to the input of the amplifier at high frequencies.

FIELD

Embodiments of the present invention may relate to circuit design. Moreparticularly, embodiments of the present invention may relate to biascircuits.

BACKGROUND

In cellular telephones and other communication devices, radio frequency(RF) power amplifiers are typically used to amplify RF signals prior totransmission. These RF power amplifiers may generate an output power inthe range of 1 mW to 3 W. In such devices, linear amplification may bedesired to prevent signal distortion. Efficiency is also aconsideration, especially for mobile devices such as cellulartelephones, due to the limited quantity of energy stored in theaccompanying battery.

Efficiency and linearity are often competing considerations. When highefficiency is important, a low amplifier transistor bias current may bechosen, thereby increasing battery life and talk time. This generallyresults in acceptable distortion at low to moderate power levels, butcreates unacceptable distortion at high power levels. When highlinearity is important, a larger transistor bias current may be chosen,reducing distortion to an acceptable level even at high power levels.The high bias current may also be required to obtain the maximum outputpower from the amplifier output transistor. However, the high biascurrent may reduce battery life and talk time, particularly at low powerlevels.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and a better understanding of the present invention maybecome apparent from the following detailed description of arrangementsand example embodiments and the claims when read in connection with theaccompanying drawings, all forming a part of the disclosure of thisinvention. While the foregoing and following written and illustrateddisclosure focuses on disclosing arrangements and example embodiments ofthe invention, it should be clearly understood that the same is by wayof illustration and example only and the invention is not limitedthereto.

The following represents brief descriptions of the drawings in whichlike reference numerals represent like elements and wherein:

FIG. 1 shows a mobile telephone constructed in a manner to includeembodiments of the present invention;

FIG. 2 is a circuit diagram of an RF amplifier and bias circuitaccording to an example embodiment of the present invention;

FIG. 3 is an equivalent circuit diagram of the bias circuit shown inFIG. 2;

FIG. 4 is a circuit diagram of an RF amplifier and bias circuitaccording to an example embodiment of the present invention; and

FIG. 5 is a graph showing inductive behaviour of impedance (Z_(out))relative to frequency according to an example embodiment of the presentinvention.

DETAILED DESCRIPTION

In the following detailed description, like reference numerals andcharacters may be used to designate identical, corresponding or similarcomponents in differing figure drawings. Further, in the detaileddescription to follow, example sizes/models/values/ranges may be givenalthough embodiments of the present invention are not limited to thesame. Well-known power/ground connections to integrated circuits (ICs)and other components may not be shown within the FIGs. for simplicity ofillustration and discussion. Where specific details are set forth inorder to describe example embodiments of the invention, it should beapparent to one skilled in the art that embodiments of the presentinvention can be practiced without these specific details.

Embodiments of the present invention may provide a bias circuit topresent a controlled impedance to an input of an amplifier to improvelinearity of the amplifier. The bias circuit may control the impedancesuch that a low impedance is presented to the input of the amplifier atlow frequencies and a high impedance is presented to the input of theamplifier at high frequencies. Embodiments of the present invention maybe used in RF circuits such as for mobile telephones or networks (suchas an RF wide local area network (WLAN))

In contrast, disadvantageous RF bias circuits may include a currentmirror and a feed/isolation resistor. For these circuits, the impedanceat DC and all frequencies may be approximately equal to a value of thefeed resistor (such as 5K ohms). That is, a value of 5K ohms may bechosen for the feed resistor to minimize noise and avoid interactionwith a matching network (loading).

Embodiments of the present invention may provide a CMOS bias circuitthat provides a low impedance at DC and a modulation bandwidth of RFcircuits while having a high impedance at RF frequencies (such as 1 GHzor greater). This may be particularly useful and important since a lowimpedance at the modulation bandwidth may improve a third-orderintercept point (iIP₃), which may in turn result in lower third-orderdistortion. Disadvantageous low noise amplifiers (LNAs) may employoff-chip inductors and low impedance bias circuits so as to terminate(or reduce) tone difference frequencies. Otherwise, the iIP₃ can belarger, varying with tone spacing, and/or result in asymmetrical iIP₃.However, the use of an off-chip inductor may be undesirable in thesedisadvantageous arrangements for cost and I/O requirements (i.e., anon-chip inductor may be large, increasing cost, and may degradeperformance due to its relatively low Q). Another biasing approach mayuse a low impedance bias circuit having a small-valued bias feedresistor. This may improve linearity, but at the expense of degradednoise and interaction with the matching network. Additionally, lowimpedance at the modulation bandwidth may also improve linearity ofLNAs, power amplifiers (PAs), and other circuits. This may also preventa shift in a bias point of a MOSFET driven by a large voltage andresulting from a nonlinear, voltage-dependant capacitance, such as in apassive mixer (triode mixer, Maas mixer).

A low DC impedance may also be useful and important for large, shortgate length transistors where gate tunneling current is significant. Ifbiased with a resistor, the DC voltage drop across the biasing resistormay de-bias the transistor and change with drive conditions,temperature, or process. Large transistors employed for power amplifiersand RF switches may have gate currents of 100 uA–1 mA. Bias circuits maytherefore have an impedance of a few ohms or less to avoidshifting/changing the bias point significantly with drive signal, andconsume little power. This may be particularly true for linear poweramplifiers. That is, bias shifts result in a softer compressioncharacteristic, degrading linearity and efficiency.

Embodiments of the present invention may therefore provide a biascircuit to present a controlled impedance to an input (i.e, a gate) ofan amplifier to improve the linearity of the amplifier. The bias circuitmay control the impedance such that a low impedance is presented to theinput of the amplifier at low frequencies and a high impedance ispresented to the input of the amplifier at high frequencies. Embodimentsof the present invention may utilize a negative feedback loop thatincludes a non-inverting amplifier and a resistor. A frequency responseof the non-inverting amplifier within the negative feedback loop may becontoured to provide a desired response.

Embodiments of the present invention may provide a low-power CMOS biascircuit that does not degrade noise performance (or not significantly).The bias circuit may have a very low impedance at DC and the modulationbandwidth. The bias circuit may further have an impedance that riseswith frequency so that the bias circuit does not affect the RF gain andmatching. The controlled output impedance may result from shuntfeedback, high loop-gain and frequency compensation.

FIG. 1 shows a mobile telephone constructed in a manner to includeembodiments of the present invention. Other mobile telephones, devicesand embodiments are also within the scope of the present invention. Morespecifically, FIG. 1 shows a mobile telephone 10 that includes amicrophone 20, a speaker 30, a keypad 40, a display screen 50, and aradio frequency (RF) antenna 60 for sending and receiving signals from astation such as a cell tower (not shown). The mobile telephone 10 alsoincludes internal circuitry 70 powered by a battery 80. The mobiletelephone 10 may be compliant with a signal frequency and modulationstandard such as AMPS, PCS, GSM, CDMA, TDMA, DCS 1800 or some othertelecommunications standard.

The internal circuitry 70 may be coupled to the speaker 30 and themicrophone 20 for communicating with a user. The internal circuitry 70may also be coupled to the keypad 40 to receive information regardingkeypad entries made by the user. The internal circuitry 70 may also becoupled to the RF antenna 60 to send and receive identification signals,voice signals, keypad entries and other information to and from thestation.

The internal circuitry 70 may communicate with the station via RFsignals transmitted through the atmosphere. To generate the RF signals,the internal circuitry 70 may include one or more RF power amplifiers(not shown in FIG. 1) capable of amplifying RF signals.

FIG. 2 is a circuit diagram of an RF amplifier and bias circuitaccording to an example embodiment of the present invention. Otherembodiments and configurations are also within the scope of the presentinvention. As one example, the RF amplifier and bias circuit may beprovided within the mobile telephone 10 shown in FIG. 1. The RFamplifier and bias circuit may also be provided within other devices,systems or networks (such as a RF WLAN) that may utilize radiofrequencies. Further, the following discussion may refer to both an RFamplifier and a bias circuit, although both may be considered as onedevice or circuit. Further, both the RF amplifier and the bias circuitmay be provided on a single chip.

More specifically, FIG. 2 shows an amplifying transistor 200 having adrain coupled to an output node 240 and a source coupled to GROUND. Agate of the amplifying transistor 200 may correspond to an inputterminal 202 of the amplifying transistor 200. Additionally, an inductor230 may be coupled between a voltage source Vdd and the drain of theamplifying transistor 200. Although not shown in FIG. 2, an amplifyingdevice may be in the form of a cascoded transistor, as opposed to thesingle transistor 200.

An RF signal may be applied to an input node 210. The RF signal may passthrough a coupling capacitor 220 and to the input terminal 202 of theamplifying transistor 200.

FIG. 2 also shows a bias circuit 250 coupled to the input terminal 202of the amplifying transistor 200. The bias circuit 250 includes aninverting amplifier 254 in the form of a transistor (or current mirrortransistor). The inverting amplifier 254 may have a source coupled toGROUND, a gate coupled to a first end of a resistor 256 and a draincoupled to a current source 252. A second end of the resistor 256 may becoupled to the input terminal 202 of the amplifying transistor 200. Theresistor 256 acts as an isolating resistor and may have a value of 10Kohms, for example.

The bias circuit 250 also includes a non-inverting amplifier 260 and aresistor 258 coupled in series in a negative feedback loop of the biascircuit 250. More specifically, a drain of the inverting amplifier 254is coupled to a positive input terminal of the non-inverting amplifier260. A Vref signal may be applied to a negative input terminal of thenon-inverting amplifier 260. The Vref signal may establish the drainvoltage of the amplifier 254. The output of the amplifier 260 is coupledto a first end of the resistor 258 and a second end of the resistor 258may be coupled to the input terminal 202 of the amplifying transistor200. The resistor 258 may therefore act as an isolating resistor and mayhave a value of 10K ohms, for example. The amplifier 260 may have afrequency response specifically contoured to obtain a desired result.

The bias circuit 250 may be configured such that at low frequency (i.e.,at or near DC current), the impedance (shown as Z_(out) in FIG. 2)presented to the input terminal 202 of the amplifying transistor 200 isvery low. As stated above, the frequency response of the amplifier 260may be specifically contoured. For example, the frequency response maybe contoured such that a gain of the amplifier 260 remains relativelyconstant at lower frequencies so that the impedance Z_(out) may remainlow. At some point, the gain of the amplifier 260 may roll off. Forexample, at 1 MHz, the gain of the amplifier 260 may decrease while theimpedance at the input terminal 202 may become very high. Accordingly,embodiments of the present invention may present a controlled impedance(Z_(out)) to the input terminal 202 of the amplifying transistor 200 toimprove the linearity of the amplifier (i.e., the amplifying transistor200).

FIG. 3 is an equivalent circuit diagram of the bias circuit 250 shown inFIG. 2. Other embodiments and configurations are also within the scopeof the present invention. More specifically, the bias circuit 250includes a resistor 262 and an inductor 264 coupled in parallel with aresistor 266. As one example, the resistor 266 may have a value of 10Kohms, the resistor 262 may have a value of 1 ohm and the inductor 264may have a value of 1–10 uH. Other values are also within the scope ofthe present invention. In this equivalent bias circuit, at lowfrequencies (i.e., DC current), the impedance Z_(out) presented to theinput terminal 202 of the amplifying transistor 200 may correspond tothe resistor 262. That is, the impedance Z_(out) may be a very lowvalue. As the frequency increases, the impedance Z_(out) presented tothe input terminal 202 of the amplifying transistor 200 also increases.All the components shown in FIG. 3 may be provided on a single chip.

FIG. 4 is a circuit diagram of an RF amplifier and bias circuitaccording to an example embodiment of the present invention. Otherembodiments and configurations are also within the scope of the presentinvention. As one example, the RF amplifier and bias circuit may beprovided within the mobile telephone 10 shown in FIG. 1. The RFamplifier and bias circuit may also be provided within other devices,systems or networks that may utilize radio frequencies. Further, thefollowing discussion may refer to both an RF amplifier and a biascircuit, although both may be considered as one device or circuit.Further, both the RF amplifier and the bias circuit may be provided on asingle chip.

More specifically, FIG. 4 shows transistors 402 and 404 that form acurrent mirror in which transistor 402 is a reference transistor andtransistor 404 is a controlling transistor (or amplifying transistor).As one example, the transistor 404 may be an RF transistor for a lownoise amplifier (LNA) or a power amplifier (PA). If the gate length isthe same for both the transistor 402 and the transistor 404, then thecurrent 12 may be represented by

$I_{2} \cong {I_{1}\;{\frac{W_{2}}{W_{1}}.}}$Transistor 406 may be a cascode device that enhances a gain of thetransistor 402 by raising its output resistance. Transistors 408 and 410and transistors 412 and 414 form cascode common-source amplifierssimilar to transistors 402 and 406. Transistors 416 and 418 may be fedby current source transistor 420 to establish a bias voltage for thetransistors 406, 410 and 414, while keeping transistors 402, 408 and 412somewhat above a knee voltage during saturation.

Transistor 422 may form a reference side of a current mirror withtransistors 424, 426 and 428. Transistors 430, 432, 434 and 436 formcascode (common-gate) devices for the transistors 422, 424, 426 and 428.Additionally, transistors 438 and 440 establish a bias voltage for thesedevices and keep the transistors 422, 424, 426 and 428 in saturation.FIG. 4 also shows bias current sources 442 and 444.

During operation, the three cascaded common-source amplifiers 402/406,408/410 and 412/414 with cascaded current source loads may develop veryhigh loop-gain to meet the objective of very low output impedance(Z_(out)) at DC and over the modulation bandwidth (such as 1–10 MHz). Acapacitor 446 and a resistor 448 as well as a capacitor 450 and aresistor 452 may compensate the amplifier for stability and establish afrequency dependent output impedance at a gate of the transistor 404because the loop-gain decreases with increasing frequency. Resistors 454and 456 may isolate the input and output of the amplifier from thecontrolled transistor 404. The values of the resistors 454 and 456 maybe chosen to be large to minimize their noise and to set the outputimpedance at a high frequency. For example, at a low frequency, theoutput impedance (Zout) may be approximately

${Z_{out} \cong \frac{R_{3}{R_{4}}}{1 + T}},$where T is the DC loop-gain, which is equal to the product of thevoltage gains of the three cascaded common-source stages. Additionally,at high frequency when the loop-gain is very small, the output impedancemay be approximately Zout≅R₃∥R₄. Because the loop-gain decreases withincreasing frequency, the output impedance rises with frequency over arange determined by the compensation network.

FIG. 4 also shows the input node 210 to receive an RF signal. The RFsignal may pass through the coupling capacitor 220 and to a gate of thetransistor 404. FIG. 4 further shows that the inductor 230 may becoupled between the voltage source Vdd and the drain of the transistor404. The drain of the transistor 404 may be coupled to the output node240. Other embodiments for the input and output of the RF signals arealso within the scope of the present invention.

FIG. 5 is a graph showing inductive behaviour of output impedancerelative to frequency according to an example embodiment of the presentinvention. Other embodiments, graphs and data are also within the scopeof the present invention. As shown for two example points on the graph,the DC output impedance is approximately 0.1 ohms, and when thefrequency is greater than 1 GHz, the output impedance is approximately5K ohms.

Embodiments of the present invention may be provided within variouselectronic systems that include RF amplifiers. Examples of representedsystems may include but are not limited to computers (e.g., desktops,laptops, handhelds, servers, tablets, web appliances, routers, etc.),wireless communications devices (e.g., cellular phones, cordless phones,pagers, personal digital assistants, etc.), computer-related peripherals(e.g., printers, scanners, monitors, etc.), entertainment devices (e.g.,televisions, radios, stereos, tape and compact disc players, videocassette recorders, camcorders, digital cameras, MP3 (Motion PictureExperts Group, Audio Layer 3) players, video games, watches, etc.), andthe like.

Any reference in this specification to “one embodiment,” “anembodiment,” “example embodiment,” etc., means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention. Theappearances of such phrases in various places in the specification arenot necessarily all referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with any embodiment, it is submitted that it is within thepurview of one skilled in the art to effect such feature, structure, orcharacteristic in connection with other ones of the embodiments.

Although embodiments of the present invention have been described withreference to a number of illustrative embodiments thereof, it should beunderstood that numerous other modifications and embodiments can bedevised by those skilled in the art that will fall within the spirit andscope of the principles of this invention. More particularly, reasonablevariations and modifications are possible in the component parts and/orarrangements of the subject combination arrangement within the scope ofthe foregoing disclosure, the drawings and the appended claims withoutdeparting from the spirit of the invention. In addition to variationsand modifications in the component parts and/or arrangements,alternative uses will also be apparent to those skilled in the art.

1. A circuit comprising: an amplifying transistor having an input; and abias circuit coupled to the amplifying transistor, the bias circuitincluding a first amplifier and a second amplifier, the second amplifierof the bias circuit to provide a controlled impedance to the input ofthe amplifying transistor based on a frequency of a signal of the input.2. The circuit of claim 1, wherein the first amplifier and the secondamplifier are coupled in a negative feedback loop.
 3. The circuit ofclaim 1, wherein the second amplifier comprises a non-invertingamplifier and the first amplifier comprises an inverting amplifier.
 4. Acircuit comprising: an amplifying transistor having an input; and a biascircuit coupled to the amplifying transistor, the bias circuit toprovide a controlled impedance to the input of the amplifying transistorbased on a frequency of a signal at the input, the bias circuitincluding a first amplifier, a second amplifier, a first isolatingresistor coupled between the first amplifier and the amplifyingtransistor, and a second isolating resistor coupled between the firstisolating resistor and an output of the second amplifier.
 5. The circuitof claim 1, wherein the first amplifier and the amplifying transistorcomprise a current mirror.
 6. The circuit of claim 1, wherein the biascircuit provides a low impedance at a low frequency and a high impedanceat a high frequency.
 7. The circuit of claim 1, wherein the bias circuitis coupled to a gate of the amplifying transistor.
 8. The circuit ofclaim 1, wherein the amplifying transistor comprises a radio frequency(RE) transistor.
 9. The circuit of claim 1, wherein the amplifyingtransistor and the bias circuit are provided on one chip.
 10. A devicecomprising: a radio frequency (RF) amplifying device to amplify a signalprovided at an input; and a bias circuit coupled to the input of the RFamplifying device to provide a varying impedance at the input of the RFamplifying device, the bias circuit including a first amplifier and asecond amplifier, the second amplifier of the bias circuit to providethe varying impedance to the input of the RF amplifying device based ona frequency of the signal at the input.
 11. The device of claim 10,wherein the bias circuit provides a frequency dependent impedance at theinput of the RF amplifying device.
 12. The device of claim 11, whereinthe bias circuit provides a low impedance at a low frequency and a highimpedance at a high frequency.
 13. The device of claim 10, wherein thefirst amplifier and the second amplifier are coupled in a negativefeedback loop.
 14. The device of claim 10, wherein the second amplifiercomprises a non-inverting amplifier and the first amplifier comprises aninverting amplifier.
 15. A device comprising: a radio frequency (RF)amplifying device to amplify a signal provided at an input; and a biascircuit coupled to the input of the RE amplifying device to provide avarying impedance at the input of The RF amplifying device based on afrequency of the signal at the input, the bias circuit including a firstamplifier, a second amplifier, a first isolating resistor coupledbetween the first amplifier and the RF amplifying device, and a secondisolating resistor coupled between the first isolating resistor and anoutput of the second amplifier.
 16. The device of claim 10, wherein thefirst amplifier and the RF amplifying device comprise a current mirror.17. The device of claim 10, wherein the RF amplifying device comprisesan amplifying transistor and the bias circuit is coupled to a gate ofthe amplifying transistor.
 18. The device of claim 10, wherein the RFamplifying device and the bias circuit are provided on one chip.
 19. Asystem comprising: a transmitting device to transmit radio frequency(RF) signals; a RF amplifying device coupled to the antenna device toamplify an RF signal to be transmitted by the transmitting device; and abias circuit coupled to the RF amplifying device, the bias circuitincluding at least one amplifier to provide a controlled impedance to aninput of the RF amplifying device based on a frequency of the RF signal.20. The system of claim 19, wherein the at least one amplifier includesa first amplifier and a second amplifier coupled in a negative feedbackloop.
 21. The system of claim 20, wherein the second amplifier comprisesa non-inverting amplifier and the first amplifier comprises an invertingamplifier.
 22. A system comprising: a transmitting device to transmitradio frequency (RF) signals; a RF amplifying device coupled to theantenna device to amplify an RF signal to be transmitted b thetransmitting device; and a bias circuit coupled to the RF amplifyingdevice to provide a controlled impedance to an input of the RFamplifying device based on a frequency of the RF signal to betransmitted, the bias circuit including a first amplifier, a secondamplifier, a first isolating resistor coupled between the firstamplifier and the RF amplifying device, and a second isolating resistorcoupled between the first isolating resistor and an output of the secondamplifier.
 23. The system of claim 19, wherein the RF amplifying deviceand the bias circuit are provided on one chip.
 24. A circuit comprising:an amplifying transistor; and a bias circuit coupled to the amplifyingtransistor, the bias circuit including a first amplifier and a secondamplifier, the second amplifier to present a controlled impedance to acontrol input of the amplifying transistor based on a frequency of asignal at the control input.
 25. The circuit of claim 24, wherein thesecond amplifier presents the controlled impedance such that a lowimpedance is presented to the control input of the amplifying transistorat low frequencies and a high impedance is presented to the controlinput of the amplifying transistor at high frequencies.
 26. The circuitof claim 24, wherein a frequency response of the second amplifier iscontoured to provide the controlled impedance to the gate of theamplifying transistor.
 27. The circuit of claim 24, wherein the biascircuit further includes a first isolating resistor coupled between thefirst amplifier and the amplifying transistor, and a second isolatingresistor coupled between the first isolating resistor and an output ofthe second amplifier.